For example, in Japanese Patent Application Laid-Open No. 62-2869, a voltage equal to a voltage applied to a vibrator is applied to a reference capacitor having an electrostatic capacitance equal to that of the damping admittance of the vibrator. Currents flowing in the capacitor and vibrator are detected to obtain a difference between them, thereby detecting a mechanical branch current.
In Japanese Patent Application Laid-Open No. 1-136575, opposite-phase voltages are respectively applied to the vibrator and the reference capacitor by using a transformer with a center tap. Each of the currents is fed back to the center tap via a resistor to directly detect the mechanical branch current.
The mechanical branch current will now be described more specifically. A piezoelectric element alone or an elastic body to which a piezoelectric element is adhered has an equivalent circuit seen from a terminal of the piezoelectric element, as shown in FIG. 2. Reference symbol electric capacitance C0 denotes a damping admittance; and mass L, mechanical elastic constant C, and mechanical loss R, dynamic admittances. A current flowing in the dynamic admittances represents a mechanical branch current; and a current flowing in the damping admittance C0, an electrical branch current.
In the above-described prior art, a circuit which converts the currents flowing in the vibrator and capacitor into the voltages is required. When the resistor converts the current into the voltage, power consumption undesirably increases. Hence, a resistance value for converting the voltage into the current needs to be small, the detection voltage becomes low, and an amplifier must be used.
In order to compensate for a change in damping admittance in accordance with changes in temperature and individual difference, a complex circuit is required. For example, the capacitance needs to become variable, or the currents flowing in the vibrator and the capacitor need to be individually detected, the gain of one current is controlled, and then the difference between the currents is obtained.